The present disclosure relates to ceramic discharge arc tubes for a High Intensity Discharge (HID) lamp, such as a ceramic metal halide discharge lamp or a high pressure sodium discharge lamp and a method of making ceramic discharge arc tubes for such lamps.
Discharge lamps, such as ceramic metal halide discharge lamps, produce light by ionizing a fill such as a mixture of metal halides and mercury, or its alternative in mercury-free discharge lamps as a buffer/voltage riser material like mercury in mercury containing lamps, with an electric arc passing between two electrodes forming a discharge plasma due to ionization of the fill. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as “dose” emits a desired spectral energy distribution of visible electromagnetic radiation also called “light” in response to being excited by the electric arc.
The arc tube in a high intensity discharge lamp can be formed from a material such as fused silica also called “quartz glass” which is shaped into the desired discharge chamber geometry after being heated to a softened state. Fused silica, however, has certain disadvantages which arise from its reactive chemical as well as its thermodynamically unstable structural properties at high operating temperatures. For example, at temperatures greater than about 950° C. to 1000° C., the halide fill reacts with the quartz glass which process produces silicates and silicate halides, thus reducing the effective quantity of fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletion phenomena cause color shift over time which reduces the useful life of the lamp. Additionally, at high temperatures, transformation of fused silica from amorphous phase to crystalline phase (“re-crystallization”) also occurs, which reduces the mechanical strength and optical transmission of the discharge chamber wall.
Ceramic discharge arc tubes were developed to operate at relatively higher temperatures for improved color control, color renderings and luminous properties while significantly reducing reactions of the discharge chamber wall with the fill material. For example, it is known to employ translucent polycrystalline alumina sintered bodies that enables visible wavelength radiation to pass through and makes the body useful for use as an arc tube for high pressure sodium and ceramic metal halide discharge lamps.
In certain applications where ceramic arc tube discharge lamps are employed in a horizontal disposition, as for example in automotive headlamp applications, the arc between the electrodes creating the plasma for producing light is configured in an upwardly arched profile which causes excessive temperatures on the upper wall surfaces of the arc tube. This extremely high “hot spot” temperature and the related temperature gradients developed within the discharge chamber wall leads to excessive thermally induced mechanical stresses in the arc tube assembly. Exposure to these excessive temperature and thermal stresses has heretofore resulted in reduced lamp reliability; and, in applications such as automotive, has resulted in premature lamp failure due to crack development and propagation within the arc tube assembly, costly replacement and attendant user dissatisfaction.
Thus, it has been desired to find a way or means for preventing excessive temperatures and thermal stresses in the ceramic arc tube assemblies of arc discharge lamps and particularly where such lamps are disposed with the arc tube in a horizontal arrangement such as found for example in case of automotive headlamp applications.